The present invention relates to a device and method for dispensing pellets, such as catalyst pellets, to a delivery point, such as into a chemical reactor vessel or into the vertical tubes of a chemical reactor vessel.
Many chemical reactors are essentially a large shell and tube heat exchanger vessel, with the reaction occurring inside the tubes and a coolant circulating in the vessel outside the tubes. A chemical reactor vessel also can be a simple tank with a single volume of catalyst inside it, or it may be a single large tube. Some chemical reactions occur in furnace or reformer tubes, which may be a part of a system with 10 to 500 or more such tubes. In any of these reactor vessels, catalyst, typically in the form of pellets (and other types of pellets that are not catalyst), may be loaded into the reactor to facilitate the reaction. The pellets are replaced periodically.
The reactor tubes may be quite long, housed in a structure several stories tall, and the pellets may be transported up several stories to an elevation above the top of the tubes so they may then flow by gravity into the tubes. The pellets typically are supplied in 2,000 pound (or larger) “super sacks”, 55 gallon drums, mini drums, metal bins or plastic bags loaded in pallet-mounted cardboard boxes.
The pellets are then carefully loaded into each reactor tube (there may be several thousand tubes in a single reactor) to try to uniformly fill each tube. It is desirable to prevent bridging of the pellets in the reactor tube, because bridging can create voids or areas within a tube in which there are no pellets. Mechanical devices may be used to aid in the dispensing of the pellets.
In some cases, in a shell and tube reactor in which vertical reactor tubes are supported by upper and lower tube sheets, a template is placed over a portion of the upper tube sheet. The template has openings aligned with the tops of the reactor tubes, with the openings in the templates having a smaller diameter than the inside diameter of the cylindrical reactor tubes in order to restrict the flow of pellets into the reactor tubes to prevent bridging in the tubes. Pellets are dumped on top of the template, and operators then use their gloved hands, paddles, brooms, or rakes to spread the pellets back and forth across the template so that pellets fall through the holes in the template and into the respective reactor tubes. Moving the pellets back and forth breaks up any bridging of the pellets above the template, allowing the pellets to flow through the holes in the template and into the reactor tubes.
In other instances, loading sleeves are inserted into each reactor tube, with each loading sleeve having a top opening that is smaller than the inside diameter of the cylindrical reactor tube in order to limit the flow of pellets to prevent bridging inside the reactor tubes. Again, the pellets are dumped on top of the loading sleeves, and the operators push the pellets back and forth across the loading sleeves so that the pellets fall through the holes in the loading sleeves and into the respective reactor tubes.
Various other dispensing techniques also are known, such as the method taught in U.S. Pat. No. 3,223,490 “Sacken”, in which a tray with a plurality of downwardly extending loading sleeves is placed directly above the tube sheet, with the loading sleeves extending into respective reactor tubes. The catalyst is poured onto the tray, and then the tray is vibrated up and down vertically, shaking the pellets to break up any bridges and allow the pellets to fall through the sleeves in the tray and into the reactor tubes. The vibration of the pellets causes them to rub against and impact against each other. Catalyst is a friable material and thus is brittle and readily crumbled. It is desirable to minimize the opportunity for the pellets to rub against or impact against each other or otherwise to be abraded or crushed, because such abrasion and crushing damages the pellets and creates dust. Raking the pellets back and forth across the template or loading sleeves creates substantial abrading of the catalyst pellets, creating dust particles which may not only fall into the reactor tubes creating higher pressure drops than desirable, but which also may become airborne, creating a health hazard for personnel inside the reactor vessel. Vibrating a tray full of catalyst as in the Sacken arrangement also causes the pellets to be jostled and to rub against and impact against each other, which also produces similar results.